The present invention relates to printing systems and, more specifically, to converting high resolution bi-level data to a lower resolution contone format for initializing ink key values in a press system.
Referring to FIG. 1, a traditional print work flow can be characterized as a three phase process. In the first or "prepress" phase (100 through 114), creation of the print product and printing plates are the central focus. In the second or "print" phase (116 and 118), the printing press is adjusted for operation and the information is printed. In the third or "postpress" phase (118 and 120), the generated product is cut, folded, and bound.
Creation of the print product (step 100) can be accomplished using any convenient tool for the acquisition or creation of text, images, and line-art. The resulting product is generally represented in an output-device independent language known as a page description language (PDL) and stored in a computer readable file 102. Well known PDLs include the POSTSCRIPT and PORTABLE DOCUMENT FORMAT languages. POSTSCRIPT is a trademark of Adobe Systems, Incorporated.
Once created, the PDL file 102 is executed by a raster image processor (step 104) to generate one or more high resolution bi-level data files 106. For example, if the target press is a CMYK device, a single high resolution data file 106 including bi-level image data for each of the device's colorants--cyan (C), magenta (M), yellow (Y), and key (K) or black--is generated. Alternatively, four high resolution data files can be generated, one for each device colorant. High resolution data files 106 typically have a resolution of between approximately 1,000 dpi (dots per inch) and 5,000 dpi.
High resolution data files are used to generate (step 108) a series of printing plates 110. A plate is often referred to as a signature and can be used to image a number of different pages. A single signature can be used, for example, to produce 4, 8, 12, or 16 pages of output, where the number of pages correspond to a single sheet of paper (or other equivalent output media). High resolution data files 106 are also used as input to a process to generate (step 112) a print control file 114. The print control file 114 is in a form suitable to control the inking and other print control parameters of a printing press. One standardized format is the Print Product Format (PPF), developed by the International Cooperation for Integration of Prepress, Press, and Postpress (CIP3) organization.
Referring to FIG. 2A, a typical offset press system 200 includes a number of ink keys 202 (only one shown), a plate cylinder 204 which carries a signature, a blanket cylinder 206 which transfers the plate cylinder's 204 ink to the paper 208, and an impression cylinder 210 to provide a hard transfer surface for the paper 208. FIG. 2B shows a face-on view of the plate cylinder 202 with a single ink key 200. Area 212 represents the circumferential band on the plate cylinder 202 that receives ink from key 200 and corresponds, by way of the blanket cylinder 206, to a band of ink on the printed paper. A typical plate cylinder 202 can be between 12 inches and 40 inches long and have between 2 and 4 ink keys 200 per inch. Thus, each ink key can correspond to a region of between one-quarter to one-half inches on an output page 208.
One task of the print control file 114 is to provide the necessary information so that a press 200 can compute a proper setting for each one of its ink keys (an ink key preset value). What constitutes a "proper level" is a function of several variables including the percentage of a key's band 212 on the signature covered by ink, and the frequency of the input data 100 corresponding to that band. As defined in the CIP3 PPF, the print control file 114 includes a print control section and a preview image section. The preview image section consists of low resolution contone data for each press colorant/plate. It is this low resolution contone data which is used by a press 200 to compute ink key preset values. One conventional method to compute ink key preset values is to rasterize the PDL input file 102 directly to contone data and then average, over a region determined by the resolution of the output device (i.e., the area under a single ink key 200), the resulting pixel values. Another method to compute ink key preset values is to count the average number of "on" bits in a region, the size of which is again determined by the resolution of the output device, of the high resolution data file 106 to generate the low resolution contone data. The former approach can require significant additional processing and is insensitive to input data 100 line frequency. The latter is totally insensitive to line frequency.